The invention relates to a method and device in a size press which comprises a nip formed by a pair of rolls and through which a paper web or board passes. The rolls in the nip are provided with surface-sizing units for applying films of size or a coating agent onto faces of the rolls. Each surface-sizing unit comprises an application zone defined by a coating member, a front edge of the surface-sizing unit and a feed chamber. The coating agent, e.g., size, is fed under pressure out of a feed pipe into the feed chamber in a direction which is transverse to the machine direction through holes or corresponding nozzles formed into a wall of the feed pipe. The coating agent flows into the application zone from the feed chamber through a throttle placed between the feed chamber and the application zone. The application zone is opened toward the face of the roll onto which the size or coating agent is being applied.
In surface sizing of paper and board webs, a so-called short-dwell technique is commonly employed. In this technique, a thin size film is applied by a surface-sizing unit to the faces of rolls in a size press. The size film is transferred onto the paper or board in the roll nip. Generally either a blade or a spiral-grooved bar, or further developments of same, are used as a metering unit for the size film.
When smoothing the coating agent on the paper or board, possibly by means of a blade in a blade-coating technique, the thickness of the size film and, thus, the moisture profile of the web can be regulated by varying the blade pressure. When a spiral-grooved bar is used, e.g., in a bar-coating technique, the thickness of the size is usually regulated by varying the groove profile on the bar. However, in size presses, one of the most difficult problems has been exactly how to provide a uniform size or coating film on the roll face and how to profile the size film.
In the blade-coating technique as discussed above, the thickness of the size film is usually regulated by pre-stressing and regulating the pressure in a loading hose arranged behind the blade. In the bar-coating technique as well, attempts have been made to affect the thickness of the size film by regulating the pressure in the loading hose arranged behind a bar cradle which supports the coating bar. By regulating the pressure in the loading hose in either technique, it is possible to regulate the bending of the blade or loading of the bar against the roll face in a very convenient manner.
However, a loading hose generally lowers the precision of profiling, but nevertheless, it is often used because of its easy adjustability. The loading hose is placed between a particular profile rib and the coating blade or the cradle of the coating bar in the blade-coating or bar-coating technique, respectively. In this manner, the profiling is carried out by adjusting adjustable spindles which deflect the profile rib in the desired way. In such a case, the profiling is not particularly precise because the pressure is distributed evenly in the loading hose and therefore the movements of the adjusting spindles are not transferred to the coating member as such.
In size presses, the size is passed into the size feed chamber through a size feed pipe arranged transverse to the machine direction. The size feed pipe is provided with densely spaced perforations in order that the size should be introduced into the size feed chamber as evenly distributed as possible. Attempts have been made to eliminate any transverse flows out of the size feed chamber as completely as possible so that the profile of the size film should become as uniform as possible.
In order to provide an even size or coating film, in the prior art devices, a double flow distribution throttle is used. In such devices, a first throttle is formed by holes formed into the size feed pipe. A second throttle is formed by a throttle plate arranged in the size feed chamber. The size feed chamber is often provided with ribs which are generally arranged to face the holes in the size feed pipe. The ribs extend very close to the holes in the size feed pipe so that the ribs optimally guide the size to the application zone and also eliminate any transverse flows.
A significant drawback of such prior art devices has been the high cost of construction, because the size feed chamber must be constructed from acid-proof and polished stainless steel. Further, such devices should be constructed so that there should not be any dead-end pockets between the throttles, i.e. between the holes in the size feed pipe and the throttle plate. The reason for this particular construction is that it is extremely difficult to clean such pockets. To achieve this goal, the second throttle, i.e. the throttle plate, is arranged to pivot. However, it is another drawback that such a construction is very difficult and expensive to manufacture.